Building Better Cities: Evaluating the Effect of Circular Economy City Construction on Air Quality via a Quasi-Natural Experiment

Table 5

The regression results of the robustness tests.

Variables	The annual maximum value of PM2.5	Other benchmark factors	Heihe-Tengchong line	Other environmental policies
	(1)	(2)	(3)	(4)
(0.7305)	(0.6028)	(0.5994)	(0.5991)
“Two-control area”  time trend		−0.1202
		(0.0734)
Capital cities  time trend		−0.0923
		(0.0996)
Special economic zone  time trend		−0.0519
		(0.2134)
Weather control variables	Yes	Yes	Yes	Yes
City control variables	Yes	Yes	Yes	Yes
City-fixed effects	Yes	Yes	Yes	Yes
Year-fixed effects	Yes	Yes	Yes	Yes
Observation	1608	1608	1608	1608
R ^2	0.4185	0.4595	0.4582	0.458

Note: (1) The regression result in Column 1 is the annual maximum value of , and the regression results in Columns 2–4 are . (2) Column 1 controls the annual maximum value of of the city. (3) Column 2 further controls the time trend of the original characteristics of the city on the basis of the baseline regression. The characteristics of the city include whether the city is a pilot city of the “dual control area,” a provincial capital city, or a particular economic zone city. (4) Column 3 controls the influence of the difference between the left and right sides of Heihe-Tengchong Line on the estimation. (5) Column 4 controls other location-based environmental policies. (6) The control variables are the same as the benchmark regression equation. (7) The symbols , , and represent a significance level of 10%, 5%, and 1%, respectively.